1. Field of the Invention
This invention relates to fuel ignition systems of the pilot ignition type and more particularly, to a control arrangement for use in such systems for providing an interlock on start-up under certain failure conditions.
2. Description of the Prior Art
In known fuel ignition systems of the pilot ignition type, a pilot valve is operated in response to the closure of thermostatically controlled contacts to supply fuel to a pilot outlet for ignition by a suitable igniter to establish a pilot flame. A pilot flame sensing circuit detects the pilot flame and effects the energization of a main valve which supplies fuel to a main burner apparatus for ignition by the pilot flame.
Typically, the operation of the main valve is controlled by a relay of the flame sensing circuit which has normally open contacts connected in the energizing path for the main valve to maintain the main valve deenergized until a pilot flame is established. When a pilot flame is established, the flame sensing circuit energizes the relay which closes its contacts to connect the main valve to an energizing circuit to permit the main valve to operate.
After the heating demand has been met, the thermostatically controlled contacts open to effect deenergization of the fuel valves to extinguish the flame. The flame sensing circuit responsively causes the relay to be deenergized, opening its contacts to disconnect the main valve from the energizing circuit in preparation for the next heating cycle. However, should the relay contacts which control the energization of the main valve become welded together following a successful ignition cycle, then, when the relay is deenergized, the main valve remains connected to the energizing circuit and will be energized when the thermostatically controlled contacts close in response to the next call for heat, even though a pilot flame is not established. Similarly, the main valve will also be connected to the energizing circuit for a circuit failure which permits the relay of the flame sensing circuit to be energized in the absence of a pilot flame. For such failure conditions, both the pilot valve and the main valve will be energized when the thermostatically controlled contacts close, permitting fuel to emanate from the pilot outlet and the main burner, an undesirable condition.
Various interlock arrangements have been proposed in the prior art, as exemplified by the U.S. Pat. Nos. 3,449,055 to J. C. Blackett, 3,644,074 to P. J. Cade and 3,709,783 to J. S. Warren, in which the energization of the fuel valves of the system is dependent upon the sequential operation of relays. In the systems disclosed in the patents referenced above, the energization of the pilot valve is effected in response to operation of a control relay which can be energized only if the flame relay is deenergized. Thereafter, the energization of the main valve is effected in response to the operation of a flame relay when a pilot flame is established, but only if the control relay is energized.
While under normal conditions the interlock circuits disclosed in these patents are effective to guard against the welded contact failure referred to above, the control relay and/or the flame relay may be energized inadvertantly following a failure of a solid state control device of the electronic circuits, permitting energization of the main valve in the absence of a pilot flame.